This invention relates generally to temperature compensation circuitry and, more particularly, to temperature compensation of surface acoustic wave devices.
As is known in the art, components generally have a characteristic which varies with temperature. As is also known, surface acoustic wave devices (SAW devices) are used in a variety of applications such as a delay element for oscillator circuits. SAW devices may be fabricated as a delay line or resonator, for example, for use in such oscillators, as well as filters and pressure transducers. Generally, a SAW device includes a pair of transducers, with each transducer having a set of conductive members which are disposed on a common piezoelectric substrate. At least the surface of said substrate supports surface wave propagation. As a delay line, for example, an electrical signal is coupled to a first one of the transducers and in response to such signal, surface waves are launched. These surface waves propagate along the surface of the piezoelectric substrate and are received by the second one of the transducers. At this second transducer an electrical signal, a replica of the original signal, is produced in response to such surface waves. The time between launching of the surface waves at the first transducer and arrival at the second transducer provides a predetermined delay. When a SAW device is used in an oscillator application, the SAW device is generally used as the delay element in a feedback loop of an amplifier. The SAW device therefore provides the requisite phase shift characteristics to an input signal fed to the input of the amplifier. Thus, if the gain of the amplifier is greater than the losses in the feedback loop, and the input signal is in phase with respect to the output signal, then positive feedback is provided around the amplifier and the amplifier will oscillate at the frequency for which the input signal is in phase with the output signal. Also commonly disposed in the feedback loop of the amplifier is a variable phase shifter. The variable phase shifter, in response to an input control signal, provides an output signal having a predetermined phase variation with respect to the phase of an input signal. One type of variable phase shifter commonly employed in SAW oscillators is a varactor diode coupled in series with an inductor. A control bias signal is fed to the varactor diode to vary its capacitance and hence the phase characteristic of the variable phase shifter.
In many applications for SAW devices, particularly with respect to applications involving oscillators, the SAW device is used because it is a relatively stable delay element. Many different types of piezoelectric materials may be employed with SAW devices. However, since the surface wave velocity characteristic of the piezoelectric material as well as the propagation length between the transducers generally vary as a function of variations in the temperature of the material, the phase shift or delay characteristic of the SAW device will also vary with temperature. One of the most common types of substrate materials employed with SAW devices is ST or rotated ST-cuts of quartz. ST or rotated ST-cuts of quartz have a temperature dependent delay characteristic which is substantially parabolic. That is, over temperatures less than the so-called "turn-over temperature" of the ST or rotated ST-cuts of quartz, the delay characteristic or phase shift decreases with increasing temperature, and at temperatures greater than the turn-over temperature of the substrate material the delay characteristics or phase shift increases with increasing temperatures.
In highly stable precision oscillator applications, it is generally required to compensate for these temperature dependent changes in the surface wave velocity and propagation length of the SAW device and hence for changes in the delay or frequency characteristics of the SAW device. Otherwise, if left uncompensated, these temperature dependent variations will cause a concomitant change in the resonant frequency of the oscillator.
One type of temperature compensation scheme commonly used in oscillators employing SAW devices as delay elements involves parametric compensation of the phase of the input signal fed, via the feedback loop, to the input of the amplifier. More particularly, an active device such as a thermocouple is used to sense the temperature of the SAW device substrate (or piezoelectric material). A signal is generated from the thermocouple which is representative of the sensed temperature and this signal is then fed to an analog multiplier or a set of parametric amplifiers which provide, in response to the temperature sensor signal, an output signal having an amplitude which varies in a predetermined manner as a function of temperature. For example, in order to compensate a SAW device fabricated on ST-cut or rotated ST-cut quartz, the output signal from the multiplier will provide a signal having a quasiparabolic amplitude characteristic. This output signal provides the control signal which is fed to the varactor diode portion of the variable phase shifter described above. In response to this control signal, the capacitance of the varactor diode varies to provide, in combination with the series inductor, a phase shift characteristic which varies oppositely with respect to the phase variation generated by the temperature dependence of the SAW device. Thus, with this arrangement, the frequency of the oscillator is relatively stable with respect to temperature. This solution, however, presents several problems. The temperature compensation circuitry, that is, the active temperature sensor and the analog multiplier or parametric amplifiers increase the weight, size, power consumption, cost and circuit complexity of the oscillator. Also, due to the presence of these extra components, the reliability of the oscillator may be reduced.